The present invention relates to a fuel cell separator and a method for producing the same, and more particularly to the fuel cell separator and a method for producing the same for use in a contact current-collection and for forming a gas passage.
In a solid polymer fuel cell, a solid polymer electrolyte membrane (simply referred to as xe2x80x9can electrolyte membranexe2x80x9d hereinbelow) is used as an electrolyte. The fuel cell has been used for space development and military because of its characteristics such that it has a high output density, its structure is simple, an operating temperature is relatively low, it has quietness and the like. In the case that hydrogen is used as a fuel, the fuel-cell essentially exhausts no nitrogen oxide and no carbon dioxide, thus it has become center of public attention for use as a low-pollution power source for automobile.
FIGS. 24(a) and (b) show an example of a basic structure of a solid polymer fuel cell. A solid polymer fuel cell 1 shown in FIGS. 24(a) and (b) has such basic structure that separators 20, 20 are disposed at both sides of a unit cell 10, which is fixed by separator frames 18, 18 made of an insulating resin material, and a water cooler 26, which controls a cell temperature, is incorporated into one separator 20.
The unit cell 10 is formed by electrodes 14, 16, which are bonded on both surfaces of an electrolyte membrane 12 having a thickness of 50 to 200 xcexcm. In general, a fluoro polymer electrolyte membrane, typically represented by a perfluoro sulfonic acid membrane, is used as the electrolyte membrane 12, being known under the trade name of Nafion (registered trademark for products manufactured by Du Pont Co.).
In electrodes 14 and 16, one side of a carbon paper or a carbon cloth is coated with mixture of a carbon particle on which platinum or the like is loaded, and a perfluoro sulfonic acid polymer solution. Then, the surface with which the mixture is coated is crimped to the electrolyte membrane 12 to form a membrane-electrode assembly (MEA). The electrodes 14, 16 are composed of two layers, namely, one is a porous and hydrophobic catalyst bed (not shown) composed of a carbon particle on which platinum is loaded, and of an electrolyte, and the other is a porous diffusion layer (not shown) composed of a carbon paper and the like.
In the separator 20, so as to supply process gases to the electrodes 14, 16, which are bonded on both surfaces of the electrolyte membrane 12, there are a great number of isolated or combined projections 24, 24 (simply referred to as xe2x80x9ca projectionxe2x80x9d hereinbelow) on a flat plate-shaped separator base 22, namely, on the surface facing with the electrodes 14, 16. The projection 24 forms a gas passage 27. The separators 20, 20 collect the generated electric power and then take out it to the outside by means of a part in contact with the electrodes 14, 16; the part is formed on the upper surface of the projection 24. Furthermore, the separator 20 prevents fuel gases and oxidants gases from mixing. For the separator 20, therefore, such material as to have gas impermeability and conductivity is employed.
Then, a great number of basic structures shown in FIG. 24(a) are laminated and compressed by the given pressure, and the electrodes 14, 16 are made to be contacted with the projections 24, 24 formed on the surface of the separator 20, 20, whereby the solid polymer fuel cell 1 is produced.
In the solid polymer fuel cell having above-mentioned structure, fuel gases such as reformate gases containing hydrogen flow toward the electrode 14 (anode) and oxidant gases such as air containing oxygen flow toward the electrode 16 (cathode) at a state such that both ends of the solid polymer fuel cell are connected to a load, whereby supplied gases pass through the diffusion layer and reach to the catalyst bed. Then, a hydrogen ion generated at the catalyst bed on the anode 14 moves to the cathode 16 through an electrolyte radical in the electrolyte membrane 12, and reacts with oxygen in the catalyst bed on the cathode 16, whereby water is generated. Electric power generated during reaction collects at the projections 24, 24 in contact with the electrodes 14, 16, then is taken out to the outside through the separators 20, 20 arranged at both ends of the solid polymer fuel cell 1.
The electrolyte membrane 12 employed in a solid polymer fuel cell needs water in order to show conductivity, therefore, process gases supplied to the electrodes 14, 16 are humidified generally. Furthermore, an operation temperature of the solid polymer fuel cell is about 80 to 90xc2x0 C. Thus, the separators 20, 20 employed in a solid polymer fuel cell are required to have not only excellent gas impermeability and conductivity but also have the stable and low contact resistance even under an oxidizing water vapor atmosphere.
For the fuel cell separator, therefore, a thin plate made of dense carbon graphite that a projection is formed thereon by means of a machine work is employed generally. Furthermore, Japanese Patent Laid-Open No. Hei 4-95354 discloses a separator of which the contact resistance with a carbon matrix electrode is made to be lowered by way of depositing Au, Ta, W, Mo or the like, on the surface of a dense carbon plate.
Additionally, a metallic separator has been also proposed. Taking account of the corrosion resistance and an electric conductivity, for example, stainless steel, Ti, Cu, Al or the like, are employed as a material for a separator. Furthermore, Japanese Patent Laid-Open No. Hei 8-222237 discloses a separator that has a thin plate having a great number of projections spaced at regular intervals of a few millimeters on both surfaces; the projections are formed by processing the thin plate such as to be composed of stainless steel, cold-rolled steel, Al or the like, that are coated with carbon graphite in a process of an embossing or a dimple forming.
Dense carbon graphite has the excellent current-collecting performance and is stable even under an oxidizing water vapor atmosphere, so it is suitable for a material of a separator. Dense carbon graphite, however, is expensive and brittle, and has the poor workability performance. A cutting work was, therefore, only known as a process for forming a projection on a surface of a thin plate composed of dense carbon graphite, thus, mass production was difficult, disadvantageously.
In contrast, a metallic material has more excellent workability than dense carbon graphite, thus a deformation processing such as a press forming may be employed as a simple method for forming a projection thereon.
However, so as to form a great number of projections on a surface of a thin plate by a press forming, a die with high accuracy is required, thereby the increase of expense is induced. In addition to this, if a projection is formed by a press forming, then a thickness of a side wall of a projection is decreased, thus causing micro cracks.
In addition, when inexpensive metal such as stainless steel, cold-rolled steel, Al, or the like, is employed as material for a separator, then an oxide film is generated on a surface of a metallic separator at a state exposed to an oxidizing water vapor atmosphere, whereby the contact resistance between an electrode and a separator is greatly increased. Therefore, the inner electrical resistance is greatly increased, thus causing the energy conversion efficiency to be decreased, disadvantageously.
To overcome this problem, as disclosed in Japanese Patent Laid-Open No. Hei 8-222237, there is such an idea as to coat a surface of a metallic separator with dense carbon graphite by means of processes such as impregnation, thermal spraying, electrocoating, sputtering or the like. According to these processes, however, tightness and adhesiveness of coating are insufficient, thus resulting in poor reliability, disadvantageously. Furthermore, due to a coating process, it results in the increase of the material cost.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide a fuel cell separator, which is inexpensive and can be mass-produced, and is capable of maintaining the low contact resistance at a state that it is used even under an oxidizing water vapor atmosphere for a long time. Furthermore, the purpose of the present invention is to provide a fuel cell separator such that there is no danger of leakage of process gases.
Another object of the present invention is to provide a fuel cell separator, which can maintain the creep strength at a high level, and such that there is no deterioration in the cell performance due to a fall of a contact pressure between a carbon cloth and an electrode, furthermore, also there is no gas leakage due to a fall of a seal pressure of a gas seal portion, even under a condition that an operating temperature of the fuel cell is over 80xc2x0 C., for instance, for use in a power source for automobile.
To overcome the above-identified problems, the first aspect of the present invention resides in that the fuel cell separator is arranged so as to face with an anode formed on one side of a solid polymer electrolyte or a cathode formed on an opposite side of a solid polymer electrolyte, wherein a contact part, for use in a contact current-collection and for forming a gas passage, and formed on a facing plane between a separator base and said electrodes, is composed of Sn or Sn alloys.
According to the fuel cell separator having above-identified structure, fuel gases flow on a surface of an anode, and oxidants gases flow on a surface of a cathode, whereby the electrochemical reaction is caused between the anode and the cathode through a solid polymer electrolyte, then the resulting generated-current is collected through a contact part formed on the separator base, then is taken out to the outside. The separator of the present invention has the excellent electric conductivity and corrosion resistance because the contact part thereof is composed of Sn or Sn alloys.
Additionally because a Sn oxide has conductivity, the stable and low contact resistance can be maintained even if the separator is driven under an oxidizing water vapor atmosphere for a long time.
In this case, as a Sn alloy material, for instance, taking account of the corrosion resistance, the heat resistance and the like, a Snxe2x80x94Ni alloy, a Snxe2x80x94Fe alloy, a Snxe2x80x94Ti alloy, a Snxe2x80x94Bi alloy, a Snxe2x80x94Ag alloy, a Snxe2x80x94Sb alloy, a Snxe2x80x94zn alloy, a Snxe2x80x94In alloy, or the like may satisfactorily be employed. Alternatively, Sn containing at least two kinds of elements selected from Ni, Fe, Ti, Bi, In, Ag, Sb and Zn may satisfactorily be employed. Among them, a Snxe2x80x94Bi alloy is particularly preferable because the contact resistance of an electrode surface shows little change with increasing time.
Furthermore, it is preferable to compose a contact part, which is formed on a surface of a separator base, of a Snxe2x80x94Bi alloy containing Ag. In this case, the creep strength can be maintained at a high level and there is no fall of a contact pressure at an electrode surface, even under a condition that an operating temperature of the fuel cell is over 80xc2x0 C., whereby the excellent cell performance is maintained. Additionally, gas leakage due to a fall of a seal pressure may also be avoided.
In this case, Bi content of a Snxe2x80x94Bi alloy which forms the Sn based alloy is preferably in the range of 3 to 20 wt %, furthermore, the amount of Ag added to a Snxe2x80x94Bi alloy is preferably in the range of 0.5 to 5 wt %. If Bi content is 3 wt % or less, then change with increasing time of the contact resistance on the electrode surface cannott be suppressed sufficiently. While, if Bi content is 20 wt % or more, then such increase causes no improvement. Furthermore, if the amount of addition of Ag is 0.5 wt % or less, the creep strength can not be retained under a condition of a high temperature (more specifically, more than or equal to 60xc2x0 C.), and if the amount is 5 wt % or more, then such increase causes no improvement.
The reason why a process of adding Ag to a Snxe2x80x94Bi alloy allows the creep strength to be maintained even under a condition of a high temperature is considered that not only Bi but also an intermetallic compound such as Ag3Sn is dispersed in a Sn matrix. In stead of Ag, therefore, Cu, Al or Sb may also be applied, because these metals have the excellent electric conductivity and corrosion resistance and form an intermetallic compound with Sn.
More specifically, in case of an above-mentioned Snxe2x80x94Ag alloy, Ag content is preferably in the range of 0.5 to 5 wt %, in case of a Snxe2x80x94Sb alloy, Sb content is preferably in the range of 2 to 10 wt %, in case of a Snxe2x80x94In alloy, In content is preferably 0.1 to 2 wt %. Furthermore, in case of a Snxe2x80x94Zn alloy, the adding amount of Zn is preferably in the range of 15 to 25%.
The second aspect of the present invention resides in that the method for producing a fuel cell separator, which is arranged so as to face with an anode or a cathode formed on one side or an opposite side of a solid polymer electrolyte, comprises a step of making a contact part of Sn or a Sn alloy by a process of a die press, said contact part being arranged on a facing plane between a separator base and said electrodes, and said contact part being for use in a contact current-collection and for forming a gas passage.
As a method for forming a gas passage by means of a process of a die molding, variety of processes may satisfactorily be employed. For instance, preference is given to a process of flowing a molten-Sn alloy on a surface of a separator base, then pressing it by means of a die in that state, to form a gas passage wall; a process of flowing a molten-Sn alloy on a surface of a die, then pressing it by means of a separator base in that state, to form a gas passage wall; or a process of pressing a die for use in forming a gas passage wall against a flat surface of a separator base in advance, then flowing a molten-metal from a molten-metal injection hole in the die to a surface of a separator base.
According to the method for producing a fuel cell separator of the present invention, a fuel gas passage and/or an oxidants gases passage, and/or a part in contact with an electrode can readily be formed at a time on a surface of a separator base by a process of a die molding of a Sn or Sn alloy material. Additionally, the method of the present invention achieves higher yield and shorter processing time than a machine working. Furthermore, a die employed for the present invention is not required to have accuracy as high as that for use in a press forming, therefore, it is possible to produce a fuel cell separator at a low cost and to mass-produce the same, advantageously.
Furthermore, because the inside of the partition walls of the resulting passage is not a cavity, there is no danger of leakage of process gases, unlike the metallic separator produced from a thin plate.
Alternatively, the method of the present invention may be realized by forming a coating layer of Sn or a Sn alloy by a process of a metal-plating on a surface of a contact part formed on a facing plane between a separator base and an electrode. For example, so as to form a layer composed of a Snxe2x80x94Bi alloy on a projection of a separator base, a Snxe2x80x94Bi alloy (layer) may satisfactorily be formed by means of processes such as a molten metal-plating, an electroplating or the like. In case of an electroplating, it is required that a porous layer has to be dense by reflowing a plated layer. The reason is that if the layer is porous, as it is, then water vapor penetrates therethrough to accelerate oxidization of the separator under a working operation with the increase of the contact resistance, disadvantageously. Alternatively, Sn and Bi may respectively be plated in order on the surface, then the resulting material may satisfactorily be reflowed so as to make it dense and to form an alloy at a time. In this case, the order of a Sn plating and a Bi plating is not a problem.